I. Field of the Invention
This invention relates to presses, more particularly, to toggle operated presses commonly used in injection molding machines. The invention is directed particularly to setting relative locations of fixed and thrust or xe2x80x9cdie heightxe2x80x9d platens in such presses.
II. Description of Related Art
In injection molding machines, material to be molded is forced into cavities defined by mating mold sections. To permit relative motion of the mold sections, typically, at least one mold section is mounted to a movable platen driven by a press mechanism. In addition to moving the press member for productive use of the machine, the press mechanism, in combination with a press structure, provides the force required to overcome the separation force produced by injection of material into the mold cavities. A commonly used press mechanism for reciprocation of the movable platen is a xe2x80x9ctogglexe2x80x9d mechanism, a combination of pivoting links which produces translation and substantial mechanical advantage. In addition to such mechanisms, the press mechanism typically includes devices for setting the relative locations of the press mechanism and a fixed platen, to accommodate tooling elements (mold sections and mold xe2x80x9cbasesxe2x80x9d) having a combined thickness within a range determined by overall press size, such thickness referred to as xe2x80x9cheightxe2x80x9d or xe2x80x9cdie heightxe2x80x9d.
Although it would be possible to overcome mold separation forces by consistently imposing a mechanism maximum xe2x80x9cclampingxe2x80x9d force, the attainment of such forces requires maximum energy consumption and increases wear of machine components. Consequently, it is preferred that the xe2x80x9cclampingxe2x80x9d force be matched to the expected mold separation force. Hence, it is known to provide press mechanisms which permit setting of desired clamp forces while also permitting adjustment of die height. Examples of such mechanisms are shown and described in U.S. Pat. Nos. 5,059,253 and 5,149,471. As described in these patents, desired xe2x80x9cclampingxe2x80x9d force is produced by elastic stretch of strain rods induced by a toggle mechanism after mating mold sections are brought into contact. Typically, force at initial contact of mold sections is controlled, by, for example, torque limit control of the toggle mechanism drive motor during press closure. Once the mold sections have made contact, greater forces are permitted for further operation of the toggle mechanism to achieve the desired clamping force.
As described in the referenced patents, desired clamping force is achieved by precise setting of relative position between the press mechanism and a fixed platen on which a mating mold section is mounted. It is known, for example, to use iterative procedures requiring operation of the press mechanism and press mechanism positioning devices. As devices for positioning the press mechanism typically change position at low rates, such iterative procedures may require substantial time to complete position setting.
It is an object of the present invention to provide improved methods and apparatus for setting die height in toggle operated press mechanisms.
It is a further object of the present invention to provide a method for setting die height of a toggle operated press wherein the likelihood of repetition of setting steps is reduced.
It is a further object of the present invention to provide a method of setting die height of a toggle operated press wherein the press mechanism is operated to locate a toggle crosshead at a predetermined position required to achieve a desired clamping force.
Further objects and advantages of the invention shall be made apparent from the accompanying drawings and the following description thereof.
In accordance with the aforesaid objects the present invention provides a method for setting die height of a toggle operated press. The toggle is operated to position the toggle crosshead at a position where the mold sections will make contact and from which further extension of the toggle mechanism will generate the desired clamping force, this position is referred to as xe2x80x9crequired crosshead positionxe2x80x9d or xe2x80x9cRCPxe2x80x9d. Thereafter, the entire press mechanism is advanced toward the fixed platen until minute motion of the toggle crosshead away from the fixed platen is detected. In the event the relative location of the press mechanism prevents initial positioning of the toggle crosshead to the RCP, the press mechanism is moved away from the fixed platen a predetermined distance and the press mechanism is again operated to position the toggle crosshead at the RCP.